The present invention relates to optical pickups and, more particularly, to a small-size optical pickup capable of correcting tracking errors and focusing errors with good precision without employing expensive optical components.
High-density recording is feasible by using laser beam for read/write operation since such an operation is achieved by, in principle, focusing light on a spot having a diameter substantially equal to the wavelength of the laser beam used. Typically used in the optical recording there is used an optical disk (hereinafter referred to as "OD") as an information recording medium and an optical pickup using a semiconductor laser device as means for achieving writing to and reading out of the OD.
Pits provided in the OD and storing information signals are of about 0.9 .mu.m in size each, and lines of pits, or tracks, are arranged with a narrow pitch of about 1.6 .mu.m. Accordingly, in a practical pickup operation the detection of pits must be achieved while correcting a displacement of a track in a direction perpendicular to the tracks or a focusing deviation due to irregularities caused by revolution of the OD.
A conventional pickup employs an optical element, such as half mirror or hologram, as a beam splitter and is adapted to split light reflected from an OD to detect pits. In this case, a displacement of a track in a direction perpendicular to the tracks in the plane of of the OD is corrected by, for example, the 3-beam method in which light from one laser diode (hereinafter referred to as "LD") as a light source is split into three beams by diffraction grating and these beams are used to correct errors, while a focusing error is detected by, for example, the astigmatism method using a cylindrical lens.
Alternatively, there is developed a SCOOP (Self Coupled Optical Pickup) method as disclosed, for example, Japanese Unexamined Patent Publication No. 72688/1991, wherein light reflected by an OD is returned to an LD (such light will be referred to as "returning light") and a signal is detected utilizing a change in oscillation state of the LD due to returning light. The objective of the SCOOP method is to reduce the number of optical components, such as beam splitter and cylindrical lens, as used in the aforesaid conventional pickup, to reduce the cost, and to facilitate precise positioning.
In this SCOOP method there are used, for example, five LDs 41 to 45 which are linearly arranged as shown in FIG. 7. The central LD 43 is for detecting recorded signals, LDs 42 and 44 on both sides of LD 43 are each for detecting tracking error signals, and the outermost LDs 41 and 45 are each for detecting focusing error signals and are slightly shifted in opposite directions along the optical axis. These LDs for detecting error signals allow tracking errors and focusing errors to be detected. In detecting these signals a differential amplifier directly compares two signals respectively detected by the LDs for detecting tracking error signals with each other to output a tracking error signal, while another one directly compares two signals respectively detected by the LDs for detecting focusing error signals with each other to output a focusing error signal. These error signals cause the respective servo mechanisms to be driven.
Referring to FIG. 8, laser beam 31 emitted from an LD chip 30 (in this figure only the light emitted from one light-emitting part is illustrated) passes through an objective lens 32 and is then reflected by a recording surface of an OD 38. The oscillation state of the LD which is coupled with the reflected light is detected by a detection part (not shown). A pair of signals detected by the detection part are compared with each other and amplified by a differential amplifier 38, while another pair of signals detected by the detection part are compared with each other and amplified by another differential amplifier 39. The respective outputs of the differential amplifiers 38 and 39 serve as a tracking error signal TE and a focusing error signal FE. A signal RF representative of information recorded in the OD is directly fed to a signal-processing circuit. The tracking error signal TE and the focusing error signal FE cause the respective servo mechanisms to operate, whereby the position of the objective lens is adjusted using the combination of an electromagnetic coil 35 and a permanent magnet 36 disposed in a casing 37.
According to the foregoing SCOOP method, the detection part must detect a slight variation in steady emission state of an LD due to returning light. Where the quantity of light emitted from an LD is detected by the detection part formed of, for example, a photodiode (hereinafter referred to as "PD"), the output of the PD (corresponding to the quantity of light emitted from the LD in its steady emission state) is 200 .mu.A, while the quantity of a variation due to returning light is very small, or as small as about 2 to about 50 .mu.m. For this reason, a differential amplifier of good precision is required to compare voltages with each other of the detection part formed of the PD. Further, even if such a differential amplifier is used, there is a problem that even a slight amount of noise frequently leads to an erroneous signal.
It is, therefore, an object of the present invention to overcome the foregoing problems and to provide an optical pickup which is applicable even to the method wherein a variation, caused by returning light, of the oscillation state of an LD is detected to read a signal recorded in an OD, and is capable of detecting tracking error signals and focusing error signals with good precision.